1. Field of the Invention
The application relates to a DC-DC converter.
2. Description of Related Art
FIG. 1 shows a soft-start DC-DC converter 100 of Japanese Laid-open Patent Publication No. 10-323026. A node N101 is connected to the inverting input terminal of an error amplifier ERA101 which is provided in a DC-DC converter control circuit 102. A reference voltage e101 is supplied to a first non-inverting input terminal of the error amplifier ERA101 and a capacitor CS is connected to a second non-inverting input terminal of the error amplifier ERA101. An external operation control signal CNT is input to a charging/discharging control circuit 130.
When the DC-DC converter 100 is activated, the operation control signal CNT is changed from a low level to a high level. Switches SW101 of the charging/discharging control circuit 130 are closed and SW102 of the charging/discharging control circuit 130 is opened, in response to the high-level of the operation control signal CNT. As a result, the capacitor CS is charged by a current i101 flowing through a constant current circuit CC and an output voltage control signal VCS outputted from the capacitor CS increases gradually from 0 V. At this time, the output voltage control signal VCS is a lower one of voltages that are input to the non-inverting input terminals of the error amplifier ERA 101, and hence an output voltage Vout is controlled by the output voltage control signal VCS. Therefore, the output voltage Vout increases gradually as the charging of the capacitor CS proceeds. In this manner, a soft-start operation is performed according to the output voltage control signal VCS.
FIG. 2 shows a chattering elimination circuit 200 using a Schmitt trigger circuit. The chattering elimination circuit 200 includes a low-pass filter 201 and a Schmitt trigger circuit 202. The low-pass filter 201 includes a resistor 203 and a capacitor 204. One end of the resistor 203 is connected to an input terminal Tin and the other end of the resistor 203 is connected to the capacitor 204 and the Schmitt trigger circuit 202. The other end of the capacitor 204 is grounded. The output terminal of the Schmitt trigger circuit 202 is connected to an output terminal Tout. The Schmitt trigger circuit 202 is a circuit which has two threshold levels (an upper limit value and a lower limit value) and whose status is changed depending on either an input voltage being larger than the threshold level or the input voltage being smaller the threshold level. The difference between the upper limit value and the lower limit value is called a hysteresis voltage, and the chattering elimination ability increases as the hysteresis voltage becomes higher.
The chattering elimination circuit 200 is used for eliminating chattering of the operation control signal CNT for the DC-DC converter 100. The operation control signal CNT is input to the input terminal Tin. Waveform distortion of the operation control signal CNT is smoothed out by the low-pass filter 201, and the operation control signal CNT is input to the Schmitt trigger circuit 202. The Schmitt trigger circuit 202 inverts the output signal when the input voltage crosses the threshold levels. The output terminal Tout outputs the operation control signal CNT with eliminated chattering, and the operation control signal is input to the DC-DC converter control circuit 102.
Related techniques are disclosed in Japanese Laid-open Patent Publication No. 11-75365, Japanese Laid-open Patent Publication No. 2005-51956, and Japanese Laid-open Patent Publication No. 2006-311729, for example.